Add a "Try it live!" button that the user needs to press prior to showing the embedded iframe

[martinRenou]

cc. @davidbrochart

Problem

When jupyterlite-sphinx embeds JupyterLite in a documentation page, it can be tens of MB for the viewer to download, this can be really annoying if the viewer is not interested in testing JupyterLite but just want to see the rest of the docs.

Proposed Solution

We should add the possibility to show a button "Try it live!" that the viewer needs to press before the iframe is added to the page.

Wishlist for this feature:

• make this an opt-in or an opt-out
• make the "Try it live!" prompt customizable

[psychemedia]

Is this something that thebe+jupyterlite might support?